1. Field of the Invention
The invention relates to the field of micromachined pressure sensors and in particular to column III nitride or GaN sensors.
2. Description of the Prior Art
The micromachining of membranes which are reliably fabricated to provide an electromechanical means of providing a measurement of pressure has been limited by micromachining fabrication techniques. Large 1xcexc membranes are needed which have a deformability sufficient to generate strains which are sensible by strain FET""s. These are difficult to fabricate at the sizes and deformation characteristics needed.
The invention is an integrated microsensor for sensing pressure of a fluid comprising a bowed micromachined membrane having a peripheral edge. A substrate is coupled to at least a portion of the peripheral edge to define a microcavity between the substrate and membrane. The pressure of the fluid in the microcavity is to be sensed. An integrated strain sensor is coupled to the micromachined membrane to generate a signal responsive to deformation of the membrane and hence responsive to the pressure of the fluid in the microcavity.
The membrane is composed of a nitride of B, Al, Ga, In, Tl or combinations thereof. In the illustrated embodiment the membrane is comprised of p-type GaN.
The strain sensor comprises an integrated circuit strain sensor or strain-FET fabricated in an integrated manner with the membrane and substrate. The strain-FET comprises an AlGaN/GaN heterostructure having an AlGaN/GaN interface where deformation of the membrane is coupled as strain to the AlGaN/GaN interface.
The integrated microsensor further comprises a frame coupled to the peripheral edge of the membrane and where the frame is coupled to the substrate, wherein the frame assists in enlarging the microcavity. In the embodiment where the membrane is comprised of p-type GaN and the frame is comprised of n-type GaN. The micromachined membrane and frame are fabricated using a photoelectrochemical etching technique. The membrane is bowed by fabrication of the integrated strain sensor thereon to create stresses across the membrane.
Alternative chemistries or choices of etchants and doped nitrides can be used to form both p type or n type suspended membranes so that the frame may be either n type or p type while the membrane layer is the opposite conductivity type.
Thus it can also be appreciated that the invention is defined as a method of fabricating an integrated microsensor for sensing pressure of a fluid comprising the steps of providing an n-type nitride layer of B, Al, Ga, In, or Tl; disposing a p-type nitride layer of B, Al, Ga, In, or Tl on the n-type nitride layer; photoelectrochemically etching a selected portion of the n-type nitride layer to define a frame formed in the n-type layer and a membrane formed in the p-type layer disposed across the frame; disposing a substrate across the frame opposing the membrane to define a microcavity defined therebetween; fabricating an integrated strain sensor on the membrane; and bowing the membrane. Bowing is controlled and determined by the initial and strain proceeding steps and conditions employed during the growth of the epitaxial crystal material comprised of column III metal nitrides and their alloys.
The invention is still further defined as a method of sensing pressure in a microcavity comprising the steps of: providing the microcavity between a substrate and a membrane, wherein the membrane is comprised of a p-type nitride of B, Al, Ga, In, or Tl and wherein the substrate is comprised at least in part of an n-type nitride of B, Al, Ga, In, or Tl, the microcavity formed between the p-type and n-type layers using a photoelectrochemical etching technique; coupling a strain FET to the membrane thereby bowing the membrane; deforming the membrane by a pressure change in the microcavity; straining the strain FET by deformation of the membrane; and sensing a change in an electrical parameter of the strain FET in response to the pressure change in the microcavity.
The step of straining the strain FET by deformation of the membrane comprises the step of straining a heterojunction in the strain FET, which is an AlGaN/GaN interface in the illustrated embodiment.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.